This invention relates to reaction chambers having ingress and egress openings, and a surface to be protected from contact with components of the reaction, in particular, for use in solar energy receivers for the protection of their transparent windows.
Extensive work has been directed in recent years to the development of efficient ways to use concentrated solar radiation as the energy source driving endothermic chemical reactions, such as for example, the production of hydrogen and carbon black by pyrolysis of methane using solar energy for process heat.
One type of a solar reactor which may be used for such a purpose is a xe2x80x9csurface receiverxe2x80x9d wherein concentrated solar radiation is introduced through the receiver""s aperture into its cavity, while the reactants flow through tubes staggered in different arrangements inside the cavity. In such a type of reactor, the radiation is absorbed at the surface of the tubes, and the heat required for carrying out the reaction is transferred through the tubes"" walls to reactants flowing inside the tubes. However, such reactors are rather bulky and their working temperature is restricted by thermal limitations imposed by the tube material and the temperature gradient across the tube walls.
In an attempt to overcome these difficulties, another type of solar reactor has been developed, called a xe2x80x9cvolumetric receiverxe2x80x9d. In such a receiver, the reactants are directly introduced into the reactor""s chamber where they themselves are exposed to direct concentrated solar radiation that enters the chamber through a transparent window. The use of such reactors elimates the need for incorporating tubes, whereby the overall heat transfer efficiency of the process is increased. An example of such a solar volumetric receiver, designed for solar heating of compressed air, was described by J. Karni et al., Proc. ASME/JSME/JSES Int Solar Eng.Conf., 1: 551-556, 1995.
However, in many chemical reactions some of the reactants and/or products of the reaction are in the form of particles. This fact presents a problem when a volumetric receiver is considered for such a reaction, because particles will eventually be deposited on the surface of the transparent window of the receiver, reducing its transparency. Moreover, the radiation that will be absorbed by these particles will cause their immediate heating up, leading to the generation of hot spots at the window, and consequently to the disintegration of the window.
Many attempts have been made to overcome this problem, and the following are typical examples of such attempts described in the literature.
Litterst (Proc. 6th Inst. Symp. On Solar Thermal Concentrating Technologies, Almeria, 1992, pp. 359-369) experimented with a vertical fluidized bed reactor, having a transparent window mounted on a cylindrical wall of the reactor. Reactants are introduced in the reactor in a primary flow parallel to the window and an air curtain is provided parallel to the primary flow direction adjacent to the window""s inner surface to protect it against contact with solid particles. This attempt failed as the thin air curtain adjacent the window""s inner surface detached therefrom under the influence of the primary flow of reactants, a short distance from its entry port. An attempt to remove the window from the primary flow by mounting it on a T-type branch did not fair much better. Solid particles were transported in xe2x80x9cpulselike eruptionsxe2x80x9d from the fluidized bed towards the window. An attempt to decelerate fast particles by injecting compressed air through radially positioned tubes near the window""s inner surface showed that huge amounts of air, in the order of 50% of the primary reactant flowrate, were required to keep the window free of contact with solid particles.
A cylindrical volumetric solar reactor with a transparent window mounted adjacent a front end of the reactor""s cavity and spaced away therefrom by an aperture plane, is described in the Paul Scherrer Institute, Final Report to Bundesamt fur Energiexe2x80x94Contract EF-REN (92) 033, p. 149. Here a suspension of ZnO powder in natural gas is injected into the reactor""s cavity in a tangential primary flow adjacent a back end thereof. The products of reaction leave the cavity through a tangential outlet port located at the front end thereof. The window is kept clean of suspended particles by means of two auxiliary flows of gas, one injected tangentially at the window and one injected radially at the aperture plane. The design was optimized to minimze the auxiliary flows while keeping the window clear of particles, however, the total auxiliary gas flowrate was 83% of the primary gas flowrate. Such a high auxiliary gas flowrate can absorb the heat received by the reaction cavity and thereby interfere with the desired reaction.
A solar receiver described in WO 96/25633 comprises an axially symmetric annular chamber with an inner wall constituted by a fiusto-conical or cylindrical quartz window through which solar radiation is admitted into the chamber. A fluid mixture in the form of a particle suspension is injected into the chamber adjacent and tangentially to an end of its outer wall and the products of the reaction are withdrawn near an opposite end of the outer wall and tangentially thereto, whereby the suspension flows around the inner wall in a whirling manner. Due to the centrifugal force acting on the whirling particle suspension, contact between particles and the window is minimized. To cool the window, the inner surface of the window is swept with a particle-free pressurized fluid.
An object of the present invention is to provide a novel solution for efficient protection of a surface in a reaction chamber.
In accordance with one aspect of the present invention, there is provided a method for protecting a surface at one end of a reaction chamber having a longitudinal axis transverse to said surface, the method comprising introducing a primary flow of reactants into the chamber in a manner whirling around said longitudinal axis, and withdrawing reaction products at an opposite end of the reaction chamber in a flow along the longitudinal axis, whereby said primary flow and said flow of reaction products approximate a free vortex flow, and introducing into the chamber a secondary protecting flow directed from a periphery of said surface towards said longitudinal axis, enabling thereby a pressure created by said vortex flow to keep said secondary flow adjacent said surface substantially over its entire area.
By virtue of the method of the present invention, a negative radial pressure gradient created by the vortex flow increases steeply towards said longitudinal axis and, therefore, towards the center of the surface to be protected, acting as an anchor to pull the secondary flow from the periphery to the center as a boundary layer without separation. This allows for the protection of the surface by the secondary flow with a significantly lower flowrate than that of the primary flow.
A further advantage of the present invention is that the path length of the whirling primary flow across the chamber is substantially extended when compared with the chamber""s axial dimension, thereby contributing to achieving higher thermal and chemical conversion efficiencies, since the vortex flow provides strong mixing of the reactants. This mixing effect also helps in preventing strong local temperature gradients in the primary flow, which could lead to flow instability due to buoyancy.
The method of the present invention is particularly useful for reaction chambers wherein a reaction is carried out in which at least one component, a reactant, a product or a catalyst, is in a particulate form. The term xe2x80x9cparticulate formxe2x80x9d as used in the present application denotes primarily a solid material being in the form of powder or particles, but may relate also to materials being in the form of liquid droplets.
The secondary protective flow may be an inert gas, but preferably is one of the reactants or products, or a mixture thereof, provided that it does not contain particles and that it is not heated in the chamber to the extent that will prevent its use in protecting the transparent window as desired, or to the extent that will cause the reaction to proceed in the secondary flow to a significant degree. Although by a preferred mode of the invention the secondary flow is in the gaseous phase, it should be understood that within the scope of the present invention it may also be in a liquid phase.
In accordance with another aspect of the present invention, there is provided a reaction chamber having a surface to be protected, and ingress and egress means designed to provide the primary and secondary flows described above. In particular, the reaction chamber has a primary ingress means adapted for introducing into the chamber the primary flow along a circumference of the chamber at a location axially spaced from the surface. It is preferable in this case that the surface and the chamber are substantially symmetric about the longitudinal axis of the chamber. It is also preferable that the primary ingress means are capable of introducing into the chamber the primary flow essentially tangentially to the chamber""s circumference to achieve a whirling flow. Appropriate ingress means are therefore typically annular and may be in the form of an impeller-like ring. It may be advantageous if the primary ingress means are capable of delivering the primary flow into the chamber in the form of a substantially conical jet, flowing away from the surface. The primary ingress means may be designed to introduce the flow in a converging or diverging manner.
The reaction chamber also has secondary ingress means adapted for introducing into the chamber the secondary flow in close proximity to a periphery of the surface. It is preferable that the secondary ingress means are capable of introducing the secondary flow essentially radially relative to the longitudinal axis. Egress means for withdrawing the reaction products are preferably in the form of an outlet port located along the longitudinal axis of the chamber at its end opposite to the surface, thus promoting the contained whirling motion that approximates a free vortex flow. Preferably, the outlet port is narrow relative to the dimension of the surface to be protected. The outlet port of the chamber may be connectable to any suitable downstream equipment, e.g. conventional gas-solid separation equipment, beat-exchanger or any other equipment as known per se in the art.
A preferred embodiment of the method and reaction chamber of the present invention concerns their use in a volumetric solar receiver having a reaction chamber and provided with effective protection of a transparent window located in a wall thereof and adapted for admitting concentrated solar radiation therein. The secondary flow according to this embodiment of the invention should preferably be a poor absorber of solar radiation and is, preferably, a non-absorbing fluid. In addition to being a protective layer, when the secondary flow is introduced at a relatively low temperature into the chamber, it will cool an inner surface of the window mainly by convecting the heat therefrom, whereby thermal loads to which the window is subjected are reduced.
The window may be planar, concave or convex, or rather it may be in the form of any appropriate surface of revolution.
The reaction chamber may have an interior design capable of directing the primary flow in a desired marmer, for example, the interior wall of the chamber may be shaped so that the primary flow entering the chamber as a conical jet flows along the chamber""s interior wall. Heating of the primary flow is greatly enhanced by heat transfer from the chamber""s interior wall.
An initial widening of the chamber in the flow direction of the primary flow renders the chamber""s diameter larger than the window""s diameter and, thereby, makes the chamber""s shape closely approximate a black body radiation cavity.
The receiver""s performance, and particularly its performance during the start up of the reaction carried out therein, may be improved by introducing into the chamber additional solar radiation absorbing particles. These particles are adapted to serve as solar radiation absorbents, allowing a rapid elevation of the temperature of the primary flow in the chamber. These solar radiation absorbing particles may be introduced as a mixture together with the primary flow, or separately, via ingress means dedicated for their introduction into the chamber.
The receiver may further be provided with third ingress means in a region exterior to the receiver chamber, in close proximity of the transparent window, to introduce therein a cooling fluid in an essentially radial or tangential flow.